Cyclic nucleotide-gated (CNG) ion channels are known for their role in phototransduction in retinal photoreceptors and in odorant transduction in the olfactory epithelium (Fesenko E E et al, Nature 313, 310-313 (1985) and Nakamura T & Gold G H, Nature 325, 442-444 (1987) both of which are incorporated by reference herein.) CNG channels are also present in other brain regions and nonsensory tissues, but their physiological roles are much less clear (Kuzmiski J B & MacVicar B A, J Neurosci 21, 8707-8714 (2001); Parent A et al, J Neurophysiol 79, 3295-3301 (1998); Kaupp U B & Seifert R et al, Physiol Rev 82, 769-824 (2002); Matulef K & Zagotta W N, Annu Rev Cell Dev Biol 19, 23-44 (2003); and Biel M & Michalakis S, Handb Exp Pharmacol 191, 111-136 (2009), all of which are incorporated by reference herein.)
CNG channel activation in photoreceptors is regulated by the cytoplasmic concentration of cGMP, which binds to and opens the channel to allow influx of Na+ and Ca2+ ions. Alterations of CNG channel activity have been observed in some forms of retinitis pigmentosa, a group of inherited diseases that cause progressive degeneration of rod and cone photoreceptors (Farber D B & Lolley R N, J Neurochem 28, 1089-1095 (1977); Bowes C et al, Nature 347, 677-680 (1990); Pierce E A, BioEssays 23, 605-618 (2001); Pacione L R et al, Annu Rev Neurosci 26, 657-700 (2003); Olshevskaya E V et al, J Neurosci 24, 6078-6085 (2004); Nishiguchi K M et al, Invest Opthalmol Visual Sci 45, 3863-3870 (2004); Trifunovic D et al, J Comp Neurol 518, 3604-3617 (2010), all of which are incorporated by reference herein.) Mutations that cause elevated cGMP levels lead to prolonged channel activation and Ca2+-triggered cell death (Pierce, 2001 supra; Trifunovic, 2010 supra; He L et al, J Biol Chem 275, 12175-12184 (2000); Rohrer B et al, J Biol Chem 279, 41903-41910 (2004); and Doonan F et al, Invest Ophthalmol Visual Sci 46, 3530-3538 (2005); all of which are incorporated by reference herein. In mouse models, reduction of CNG channel activity strongly correlated with improvements in the overall progression of the disease (Fox D A et al, Eur J Ophtalmol 13, S44-S56 (2003); Paquet-Durand F et al, Hum Mol Genet 20, 941-947 (2011); Vallazza-Deschamps G et al, Eur J Neurosci 22, 1013-1022 (2005); Woodruff M L et al, J Neurosci 27, 8805-8815 (2007); and Liu X et al, PLoS One 4, e8438 (2009) all of which are incorporated by reference herein.)
The most widely used CNG channel antagonist in research, I-cis-diltiazem, is an incomplete blocker (Stern J H et al, Proc Natl Acad Sci USA 83, 1163-1167 (1986); Hashimoto Y et al, Eur J Pharmacol 391, 217-233 (2000); Haynes L W, J Gen Physiol 100, 783-801 (1992); Galizzi J P et al, J Biol Chem 261, 1393-1397 (1986) all of which are incorporated by reference herein). CNG channels are also blocked by some local anesthetics, one example being tetracaine [2-(dimethylamino)ethyl 4-(butylamino)benzoate], which is referred to herein as compound 1 (Quandt F N et al, Neuroscience 42, 629-638 (1991); Schnetkamp P P, Biochemistry 26, 3249-3253 (1987); Schnetkamp P P, J Gen Physiol 96, 517-534 (1990), all of which are incorporated by reference herein.) Compound 1 blocks CNG channels with relatively high affinity, although differently from voltage-gated sodium channels. Similarly to sodium channels, the interaction of compound 1 with CNG channels is thought to be located in the selectivity filter and the pore region (Sunami A et al, Proc Natl Acad Sci USA 94, 14126-14131 (1997); Ragsdale D S et al, Science 265, 1724-1728 (1994); Ragsdale D S et al, Proc Natl Acad Sci USA 93, 9270-9275 (1996); Catterall W A, Novartis Found Symp 241, 206-218 (2002); Fodor A A et al, J Gen Physiol 110, 591-600 (1997), all of which are incorporated by reference herein).
The CNG channels of retinal photoreceptors are non-selective cation conductances that regulate the membrane potential in response to light (Fesenko E E, et al, Nature 313, 310 (1985) and Nakamura T & Gold G H, Nature 325, 442 (1987), both of which are incorporated by reference herein.) Unlike voltage-gated potassium channels, these channels are directly activated by the binding of cGMP, and are minimally regulated by voltage. In photoreceptors, photons trigger a signaling cascade that leads to a decrease in cGMP levels and closure of channels.